Non-destructive sensing techniques using terahertz waves have been developed in recent years. They include imaging techniques for safely seeing through objects to examine them instead of X-rays. Such techniques can typically provide applications of electromagnetic waves of the above cited frequency range. Additionally, spectrometric techniques for examining physical properties of substances such as bond conditions in the inside of the substance by determining the absorption spectrum and the complex permittivity of the inside of the substance, analysis techniques for analyzing bio-molecules in the inside of substances and techniques for evaluating the concentration and the mobility of carriers in the inside of substances.
Attempts to check prohibited drugs and dangerous matters hidden in pieces of baggage and in the cloths and the bodies of passengers in airports and customs offices by means of see-through examination apparatus using a terahertz wave have been under way. Examining human bodies by irradiating them with X-rays gives rise to the problem of exposure to radiation and therefore techniques using terahertz waves are believed to be effective. Besides, non-destructive quality examinations are important on production lines in manufacturing plants. Therefore, applications of such techniques to internal examinations of ICs, detection of foreign objects in powdery materials and also detection of defects in molded plastic products are being discussed.
It is important to acquire tomographic images in such applications. Therefore, there has been proposed a technique of irradiating an object of examination with a terahertz wave pulse and analyzing the plurality of pulses produced by reflection in terms of delay time and pulse form to acquire tomographic images (see Patent Document 1: Japanese Patent Application Laid-Open No. 11-108845). The above-cited patent document describes an example of observation of the inside of a flexible disk. There has also been proposed a technique of using a reflected or transmitted wave of a terahertz wave pulse, performing a Fourier transform of the time domain waveform of the pulse, to observe the frequency spectrum in order to display how chemical samples are distributed in the depth direction in the inside of an object, which may be a medicine in most cases (Patent Document 2: Japanese Patent Application Laid-Open No. 2006-516722).
As a suitable technique for generating a terahertz wave, there is a known method of irradiating a laser beam emitted from a femtosecond laser onto a photoconductive switching device prepared by using a photoconductive film formed on a substrate and provided with an antenna that also operates as an electrode is known (see Patent Document 1). While LT-GaAs grown on a substrate at low temperature is popularly employed as photoconductive film, InGaAs, InAs or GaSb may alternatively be used.